1. Field of the Invention
The present invention relates to a method for the analysis of impurity elements in quartz glass used in the fields of semiconductor industries and optical communication or in amorphous or crystalline silicon dioxide as the starting material thereof and, more particularly, relates to a method for the quantitative analysis of the element of zirconium in natural quartz.
2. Description of the Related Art
Heretofore, analysis of the impurity elements in silicon dioxide is conducted for the quantitative analysis by the method in which silicon dioxide is decomposed with hydrofluoric acid and converted into a solution and the impurity elements in this solution are analyzed by the conventional procedure of purity analysis of a solution so as to utilize the results for the calculation of the contents of the impurity elements.
Although hydrofluoric acid has a strong power to decompose silicon dioxide, the dissolving power thereof for other metallic impurities is not always high enough so that it is widely performed in conducting the decomposition to use an acid mixture thereof with nitric acid which has a high dissolving power for metallic elements.
When it is desired to conduct analysis of an element which is subject to vaporization and dissipation in the form of a fluoride, such as boron and the like, it is also performed to add a non-volatile acid such as phosphoric acid and sulfuric acid or to add an organic additive such as mannitol capable of adsorbing and immobilizing the element.
The inventors, however, have discovered the fact that no accurate analytical results can be obtained by these conventional procedures for several of the elements in quartz glass. In particular, it has been found that no accurate quantitative analysis can be performed for the element of zirconium because this element cannot be fully converted into a solution in the course of the starting decomposition procedure of the sample.
Further, the inventors have conducted studies on an efficient means by which this undissolvable matter can be decomposed and converted into a solution and, as a result of the extensively continued investigations for the analytical method by which accurate analysis can be performed for the total amount of the impurities contained therein, have arrived at a discovery that a possibility can be obtained for the analysis of the elements such as zirconium which cannot be accurately analyzed by the conventional analytical methods for quartz glass leading to completion of the present invention.
In particular, natural quartz powders contain impurities in various forms depending on the process of geological formation thereof. It has become clear that a part of them cannot be accurately analyzed by the conventional procedures because they cannot be decomposed and converted into a solution. Moreover, it has also become clear that some of the purification procedures for the purification of the starting materials thereof cause conversion of the existing impurities into a hardly decomposable form.
With the foregoing drawbacks in view, an object of the present invention is to provide a method for the analysis of the amounts of impurities in silicon dioxide by which the total amount of the impurities contained in silicon dioxide or, in particular, zirconium in a natural quartz powder can be quantitatively determined with high accuracy.